ITT: Mysterious Deaths of Migrating Bats

There are more than 1,300 bat species. Some migrate. But for years researchers haven’t had much information about their migration patterns. Now, because of hundreds of telemetry towers and transmitters glued onto bats’ backs, a Texas Tech bat researcher is getting data about where the bats go. That could help Liam McGuire discover why hundreds of thousands of the flying mammals across North America die each year because of wind turbines.

The bat deaths could eventually affect agricultural producers. Each night in Texas, bats devour as many as a thousand tons of insects, meaning the use of fewer chemical pesticides. So if the number of bat deaths keeps rising, more pesticides might be needed.

“If we’re losing huge numbers of these bats to wind energy mortalities than that’s going to potentially have an impact on that whole community across the continent,” he says. “There are lots of insects out there that like to eat lots of plants, and there’s lots of bats out there that like to eat lots of insects.”

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McGuire is using a system of towers called Motus, which is Latin for movement or motion. About 400 telemetry towers are part of the collaborative effort. They are located along the Gulf Coast from Texas to Florida and then move up the East Coast to Maine. There are also towers around the Great Lakes. There are a few in West Texas and one in New Mexico.

The towers pick up signals from small transmitters on the bats that weigh about a third of a gram and are about a half inch long.

Picture shows a bat with the transmitter.

Credit Liam McGuire

“The system in principle can work on any animal carrying the tag,” he explains. “So, I’ve been using it with bats. There are other folks looking at using it for dragonflies—any animal that moves around that’s big enough to carry the little transmitters, the system will work for any of those.”

The Motus project began in late 2008 with five towers on the north shore of Lake Erie for a bird study. Motus relies on an array of sensors in the towers that simply wait for animals to pass within range and then share their radio telemetry data with a central repository. Before he began using the towers in 2009, McGuire says, researchers used attached transmitters but that method’s results were limited and difficult.

“Before we had this network of towers, the only way that we could really study these animals was to catch a bat, attach a transmitter, release it and then try and chase it around and follow it with a truck, holding an antenna out the window. These bats don’t tend to stay by roads and they move very quickly and so for a migratory species, that’s basically impossible. So it’s only when we started getting the ability to track these animals with these networks that we could really start getting into asking questions about what’s going on during migration,” McGuire says.

It used to be that every transmitter had a different frequency, which required hours to dial through before sending the data on to specific researchers. Now all the tracking happens on one frequency and each transmitter has a distinct digital code.

The bat deaths involve only the migratory species. So, is it the migratory aspect that makes wind turbines an issue?

“That’s the question,” McGuire says. “What the attraction and what the reason why these bats are being killed by these wind turbines, that’s still a question that’s out there. I think we’ve got parts of the question figured out, but there’s still the question of: why not these other species.”

Already there are a few hypotheses. One is that the migrating bats view the turbine towers as giant trees, which would make them attractive markers for male and female to find one another. It’s also possible that bats who migrate over long distance fly higher than those that are regional migrators.

Tower used to track migratory bats.

Credit Liam McGuire

Another is bat anatomy. Studies have shown that bats killed at wind turbines have no external injuries. The air pressure drops in the wake of a turning turbine blade.

“The papers that have come out suggest that as bats are flying along and they fly into this low pressure system, their lungs—which are basically just like ours, balloons—all the sudden there’s this rapid pressure change and it damages the tissues in the lungs and so these bats end up with what they call barrow trauma. That is potentially one reason why you get a lot of dead bats, compared to dead birds,” he says.